This invention lies in the field of rock drilling. More particularly, it is concerned with rock drilling machinery for drilling large diameter holes, of the order of up to 10 to 20 feet in diameter. Still more particularly, it concerns a rock drilling system in which the hold down force between the bit and its cutters, and the cutting face at the bottom of the hole, is created by the weight of drilling liquid pressing on the top of the bit.
In the prior art, and particularly, in the drilling of relatively small diameter holes, of the order of 10 to 20 inches, the weight on the bit required for proper cutting action of the cutters is created by dead weight, in the form of drill collars, which are attached to the bit and are supported by the drill pipe. If increased weight on the bit is required more drill collars are provided, etc.
However, in the drilling of very large diameter holes, in order to get the required force of some 40 thousand pounds per cutter, where there are a great number of cutters, because of the large cross sectional area of the bore hole, the amount of weight required might reach up into the millions of pounds, and would be impractical to supply in the conventional manner.